Reservoir pressure maintenance process



March 12, 1968 W. M. REID 3,372,747

RESERVOIR PRESSURE MAINTENANCE PROCESS Filed May 17, 1965 VV//am M //O//PV 6,45 EAN/f INVENTOR.

/Mmf/V sf IP/PA' VEL United States Patent O 3,372,747 RESERVOIR PRESSUREMAINTENANCE PROCESS William M. Reid, 4747 W. Alabama, Houston, Tex.77027 Filed May 17, 1965, Ser. No. 456,344 2 Claims. (Cl. '166-7)ABSTRACT F THE DISCLOSURE A method of controlling or maintainingreservoir pressure wherein pressurized gas recovered from the reservoiris expanded to Idrive a pump to pump water into the reservoir.

This invention relates to new and improved methods for controlling ormaintaining reservoir pressure in a subsurface reservoir from whichfluids containing useable quantities of hydrocarbons are removed, andparticularly, to methods for retarding retrograde condensation whichnormally occurs in reservoirs containing iluids within a certain rangeof temperatures and pressures when such fluids are removed therefrom.

In lrecovering hydrocarbons from certain subsurface reservoirs, it isrecognized that substantial amounts of hydrocarbons remain in thereservoir as residue due to retrograde condensation which accompaniesthe decline of reservoir pressure. While attempts have been made toarticially sustain or support reservoir pressure by gas cycling or waterilooding, these methods, as employed heretofore, have required the useof internal combustion prime movers to operate the compressors forpressurizing the gas sufficiently for injection into the high pressurereservoir or for operating pumps to inject liquids into the producedreservoir to water flood the formation.

An object of the present invention is to provide a new and improvedmethod and for supporting or maintaining pressure in a subterraneanreservoir from which hydrocarbons are removed so as to reduce retrogradecondensation in the reservoir without requiring the use of internalcombustion prime movers.

Another object of the present invention is to provide a new and improvedmethod for supporting or maintaining reservoir pressure in a producingformation wherein pressurized gas produced from the reservoir isexpanded through an expansion device to pump water or other suitablesubstances into the reservoir to maintain the reservoirs pressure.

Still another object of the present invention is to provide a new andimproved method for maintaining or supporting reservoir pressure so asto retard retrograde condensation in a producing formation by expandinggas produced from the reservoir in an expansion engine to drive a pumpfor injecting water into the reservoir from which the gas was initiallytaken.

And yet another object of the present invention is to provide a new andimproved method for restoring or supporting pressure of a reservoir fromwhich gas and liquid hydrocarbons are removed wherein a separator isprovided to separate the gas from the liquids produced from thereservoir and the gas is expanded through an expansion device to operatea pump to inject water into the producing reservoir.

A further object of the present invention is to provide a new andimproved method for restoring or supporting the pressure of a reservoirfrom which gas and/or liquid hydrocarbons are produced wherein the welleflluent is heated and the hot gas separated from the heated liquids andlthereafter expanded to operate an expansion engine to inject water intothe subterranean reservoir and heat from the heated liquids is exchangedwith unheated fluid produced from the reservoir.

Still a further object of the present invention is to provide a new andimproved method which is adapted to be used in conjunction with theconventional pressure maintenance systems such as gas cycling or waterinjection to provide an auxiliary power source for extracting energyfrom the gas produced wherein pressurized gas withdrawn from thereservoir is expanded in an expansion device which is adapted to operatea pump apparatus to pump liquids into the reservoir.

Still another object of the present invention is to provide a new andimproved method for withdrawing pressurized gas from a reservoir andpartially expanding the pressurized gas through an expansion device todo mechanical work. y

The preferred embodiment of this invention will be describedhereinafter, together with other features thereof, and additionalobjects will become evident from such description.

The invention will be more readily understood from a reading of the`following specification and by reference to the accompanying drawingsforming a part thereof, wherein an example of the invention is shown,and where- 1n:

FIG. 1 illustrates a low diagram of the apparatus and process of thepresent invention for use with high pressure reservoirs wherein the welleffluent is not heated; and

FIG. 2 illustrates a tlow diagram of an embodiment of the apparatus inprocess of the present invention for use with reservoirs in which thewell eluen-t is heated.

The process of the present invention which is shown schematically in theHow diagram of FIG. 1 is designated there generally by the letter A.Such process is used for maintaining or supporting the pressure in areservoir R containing a fluid and gaseous mixture within a range oftemperatures and pressures such that retrograde condensation ofliqueliable hydrocarbons occurs when reservoir pressure decreases. Suchfluid and gas are removed from the reservoir R through one or more wellsP and other fluids are injected into the reservoir through one or moreinjection wells I.

With the method of this invention, reservoir pressure is maintained orits decline is retarded by expanding gas taken from the reservoir Rthrough an expansion device E which is adapted to operate a pumpingdevice B to inject water into the reservoir R via the injection well orwells I.- As will be shown by example hereinafter, the pressureextracted from the uids produced will, in many instances, be sufiicientto pump enough water into the reservoir R to maintain the fluid pressuretherein, and in `some instances, to actually increase reservoirpressure.

Referring more specically to the flow diagram shown in FIG. 1 of thedrawings, the gaseous mixture to be'- processed is produced or allowedto tlow out of the reservoir R through the producing well or wells P ina manner well known in the art. Such gaseous mixture is normallyconducted through a pipe or conduit 10 to a separator or dehydrator 12positioned on the earths surface at some convenient location relative tothe well P. It will be appreciated that with this invention, anindividual separator 12 may bc used for each producing Well P or aplurality of wells P may be connected so as to pass their Veffluentthrough a single separator 12, as desired.

In the separator or dehydrator 12, the 4water Aand liquid hydrocarbonsare removed or separated from the gas, the liquids being transmittedthrough the pipeline 14 to appropriate storage or disposal (not shown)and the gas being conducted to the expansion device E through a suitableconduit or pipeline 17.

The expansion device E, which is illustrated schematically in FIG. 1 ofthe drawings, may be a gas turbine or other suitable type of expansionengine in which the gas is allowed to expand. The gas emerging from suchexpansion engine E at a reduced temperature and pressure is conductedvia the pipe or other suitable conduit 19 to a separator 20 in whichsuch other hydrocarbons as may remain in the gas are separated therefromfor delivery to storage facilities (not shown). The gas emerges from theseparator 20 through the outlet 21 from which it is conducted via apipeline to a cycling plant or other processing facility or sales, asmay be desired.

As shown, the expansion device or turbine E is connected to a pump Bwhich is connected to a suitable source of liquid such as salt water(not shown) by means of an inlet line or pipe 30. The e'iuent of thepump B is transmitted 4through the pipeline 31 which conducts the liquidfrom the pump B to the injection well or wells I which, as shown in thedrawings, penetrates the formation or stratum in the reservoir R fromwhich the gaseous mixture is produced. Preferably, the arrangement ofthe producing well or wells P and the injection wells I relative to thereservoir structure R is as illustrated in FIG. 1, namely, the injectionwell I is bottomed in a portion of the reservoir structure which isdeeper, sometimes by as much as a thousand feet or more, than theportion of the reservoir for-mation from which the gas or other fluidsare being taken by the well P. Also, in most instances, it is preferredto have a dry gas bank between the water W in the reservoir R andreservoir uid F, as this will achieve the best liquid recovery from thereservoir R.

By way of example of the operation of the process of the presentinvention, let it be assumed that the following conditions obtain withrespect to the reservoir R shown in FIG. 1 of the drawings. Assume thereservoir depth is ten thousand feet, the reservoir pressure is 7,000p.s.i., the molecular weight of the reservoir lluid is 20, the reservoirtemperature is 300 F. and that the super cornpressability of thereservoir fluid is 1.15. With these factors, it can be calculated thatthe density of the reservoir fluid is 14.9 pounds per cubic foot.

In actual practice, a wellhead pressure of 4,000 p.s.i. and atemperature of 200 F. is not uncommon and with long ow lines, may be aslow as 3,000 p.s.i. and 80 F. With sales delivery pressure at 1,000p.s.i., then with the foregoing data, the work available will be 46B.t.u. per pound of reservoir uid, and assuming 80% mechanical eiciency,the work available from each cubic foot of reservoir fluid will be 545B.t.u.

Comparing this with the work required to pump one cubic foot of water,when the liquid has a specific gravity of 1.0, the formation facepressure drop is 1,000 p.s.i., the line loss drop is 500 p.s.i., and themechanical efficiency of the pump is 75%, it can be calculated that1,030 B.t.u. of work will be required to pump each cubic foot of waterinto the injection well I as shown in FIG. 1 of the drawings. Therefore,in this example, by expanding gas taken from the reservoir R asdescribed herein above, sufcient liquid can be pumped to lill 53% of thereservoir voidage.

It should be noted, however, that in making these calculations, variousassumptions have been indulged which are more conservative than wouldnor-mally be found in many cases. For instance, water volume does nottake into account an increase in water temperature; both the injectionwell I and the producing well P are assumed to be of the same depth,which occurs only occasionally; an increase in the specic gravity of thewater will decrease the work, and in many instances, salt water, havinga specic gravity of 1.04 or greater, is available for water injectionpurposes. Thus, it can be appreciated that even with less than idealconditions, with the method or apparatus of the present invention,reservoir pressure can be supported or maintained and retrogradecondensa tion thereby retarded. However, in the event of favorablecircumstances, the gas may be only partially expanded to operate a pumpto pump liquid into the reservoir to make up a voidage in the reservoirand the balance of the gas pressure reduction may be used to achieverefrigeration of the well effluent, or for power recovery for other usessuch as generation of electric power, mechanical refrigeration or otherpumping needs. And in some situations, it may be advantageous to useexpansion from one reservoir to support or increase the pressure inanother reservoir.

In the above illustration, the reservoir fluid molecular weight ischaracteristic of gas distillate or retrograde reservoirs; however, themethod and apparatus of this invention is applicable to liquidreservoirs, commonly termed crude oil reservoirs, if suicient gas can beseparated from the liquids produced from a crude oil reservoir to powerthe expansion device E. If the separated gas is not sucient to power theexpansion device E, the entire well effluent could be heated, the vaporportion separated from the liquid and the vapors used to power theexpansion device E as will be shown and explained in detail hereinafter.

Referring now specifically to a flow diagram in FIG. 2 of the drawingswhich illustrates the method and apparatus of the present invention forheating well efuent, it will be seen that the gas mixture to beprocessed is taken from the producing well or wells P in a manner wellknown in the art. Such fluid is then conducted through a pipeline orconduit 40 to a separator 41 in which the water and some liquidhydrocarbons may be separated from the gas or, if desired, the liquidhydrocarbons may be retained with the gas and the other liquidsseparated from the hydrocarbon fluid. The separated water or liquidhydrocarbons and water emerge from the separator 41 through the conduit42 and are conducted thereby to suitable storage or further processingfacilities (not shown. The separated gas mixture emerges from theseparator 41 and then passes through a suitable conduit or pipe 43 to aheat exchanger 45 where the gas stream takes up heat from a liquid andgas stream separated out of the same gas stream at a later stage in thisprocess as will be explained. Upon emergence from the heat exchanger 45,the gas is then heated in a heater or furnace 47 which may be directlyor indirectly red, as desired.

The heated gaseous mixture is then conducted to a separator 50 in whichthe hot gas is separated from the hot liquids. The hot gas passes to theexpansion engine E and the heated liquid is then conducted through aconduit 51 to the heat exchanger 45 where heat is given up to theincoming stream of gas coming from the separator 41 as describedhereinabove. After giving up its heat, the fluid passes through aseparator 60 and thence to storage or sales facilities (not shown).

The heated gas is expanded in the expander E which is connected to drivethe pump B' for pumping water or other suitable liquid from a supplysource (not shown) through a pipeline or conduit 53 to the injectionwell or wells I.

After having been expanded in the expansion engine or apparatus E', theheated gas is preferably conducted through a conduit 55, which, asshown, connects with the conduit 51 for conducting the heated gas to theheat exchanger 45 where such heated gas also gives up heat to theincoming gas passing `through the heat exchanger 45 from the separator4I.

As shown in dotted line in FIG. 2, an alternate course or conduit 63 maybe provided, if desired, for conducting the expanded gas directly fromthe expander E to the separator 60 rather `than going through the heatexchanger 45. In'the separator 60, the liquids and gases are separated,the gas being conducted to sales, cycling, dehydration, or the like, viaa conduit 61 and the liquids going to storage or processing via asuitable conduit 62.

In some situations, it may be desired to heat the entire well eluentprior to separating liquid constituents from the gas. In that event, theseparator 41 may be eliminated from the flow pattern between the well Pand the heat exchanger 45, and the reservoir fluid would be heated inheat exchanger 45 and in the furnace 47 prior to having liquids such aswater and liquid hydrocarbons removed therefrom. After being heated, theWell eiiiuent would be conducted through a conduit or pipe 48 to theseparator 50 where the heated gas would be separated from the liquids,the gas going to the expander E and the liquids going through theconduit 51 to the heat exchanger 45.

Assuming that the form of the invention shown in the flow diagram ofFIG. 2 is used for heating the well effluent and that with otherconditions being as assumed in the foregoing example, that the welleffluent is heated to 500 F., the Work available from expansion of gasin the reservoir liuid is increased to 1,250 B.t.u. per cubic foot ofreservoir iiuid. In the example cited, this Well permit 1.2 cubic feetof Water to be pumped into the injection Well or wells I for each cubicfoot of reservoir uid removed from the reservoir via the producing Wellor Wells P, thus producing an increase in the reservoir pressure. Or, ifdesired, the system could be operated with a liquid pressure drop ofapproximately 800 p.s.i. in liquids being pumped into the injection wellor wells I' and yet maintain full reservoir pressure.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof and various changes in the size,shape, and materials, as Well as in details of the illustratedconstruction, may be made within the scope of the appended claimsWithout departing from the spirit of the invention.

What is claimed is:

1. A method of producing a gaseous mixture containing liqueablehydrocarbons from a reservoir in which such gaseous mixture is confinedat temperatures and pressures within a range such that retrogradecondensation of liqueiiable hydrocarbons occurs when reservoir pressuredecreases which method comprises:

(a) removing the gaseous mixture containing liquefiable hydrocarbonsfrom the reservoir;

(h) heating such mixture;

(c) separating the liquid from the gas constituents of said mixture;

(d) expanding the separated gas to mechanically pump liquid into thereservoir from which said mixture was removed;

(e.) combining the expanded gas with the heated, separated liquid; and

(f) exchanging heat from the combined expanded gas and heated liquidwith the unseparated gaseous mixture to heat said gaseous mixture andcool said combined expanded gas and heated liquid mixture.

2. A method of producing a gaseous mixture containing liqueablehydrocarbons from a reservoir in which such gaseous mixture is confinedat temperatures and pressures Within a range such that retrogradecondensation of liqueable hydrocarbons occurs when reservoir pressuredecreases comprising:

(a) removing the gaseous mixture containing liquefiable hydrocarbonsfrom the reservoir;

(b) separating the liquid from the gas constituents in such mixture;

(c) heating `the separated gaseous portion;

(d) separating liquids from such heated gaseous portion;

(e) expanding said heated gas portion to perform mechanical work;

(f) using such mechanical work t0 pump liquid into the reservoir fromwhich the gas mixture was removed;

(g) combining the expanded hot gas with the separated heated liquids;

(h) exchanging heat from said combined expanded gas and heated liquidswith the unheated gaseous portion to heat said gaseous portion and tocool said combined liquids and expanded gas; and

(i) separating said cooled gas and liquids.

References Cited UNITED STATES PATENTS 1,658,910 2/1928 Trumble.

2,309,075 1/1943 Hill 166--7 2,617,484 11/1952 Swearingen 166-73,149,668 9/1964- Arendt 166-7 OTHER REFERENCES Uren, Lester C.:Petroleum Production Engineering: Oil Field Exploitation. New York,McGraw-Hill, 2nd ed., 1939, pp. 280-282 and 482.

CHARLES E. OCONNELL, Primary Examiner. IAN A. CALVERT, AssistantExaminer.

